PhD Dissertation Defense: Alexandria Hounshell

The PhD Dissertation Defense of Alexandria Hounshell will be presented by the University of North Carolina at Chapel Hill Department of Marine Sciences and Institute of Marine Sciences. This event is scheduled for Friday, March 15th, at 9:00 am in room 222 of UNC’s Institute of Marine Sciences in Morehead City, NC. This seminar will be simultaneously broadcast live to both UNC’s Department of Marine Sciences in the 3rd floor conference room (3204) of Murray Hall in Chapel Hill and online via Zoom (Meeting ID: 252-726-6841).

Title: Tracking the sources and fates of fluorescent organic matter in the eutrophic Neuse River Estuary, North Carolina

Abstract: Eutrophication is defined as ‘an increase in the rate of supply of organic matter (OM) to an ecosystem’. In estuaries, this can take two forms: as an increase in the amount of terrestrial, allochthonous OM and as an increase in autochthonous OM produced in situ. The goal of this dissertation was to use spectrofluorometry, as excitation emission matrices (EEMs), and other measures of OM quantity and quality, to constrain the OM pool in the eutrophic Neuse River Estuary (NRE) and to assess how human activities are altering the quantity and quality of OM.
EEMs can be coupled with the statistical decomposition technique, parallel factor analysis (PARAFAC), to identify broad classes of fluorescent OM (FOM). The first chapter focused on identifying a fluorescent dissolved OM (FDOM) estuarine processing signal that could be incorporated into a source-based PARAFAC model to track the sources of FDOM through the NRE. The second chapter used multivariate statistical techniques to identify sources of FOM in the NRE. Results suggest the FDOM pool is composed of terrestrial, humic-like OM while the fluorescent particulate OM (FPOM) pool contains both terrestrial, humic-like and autochthonous OM.
In the final chapters, we focus on how climatic and anthropogenic activities alter OM in the NRE. First, a dissolved organic carbon (DOC) source and sink term, under different riverine discharge conditions, was calculated to assess when the estuary acts as a processer versus a pipeline for the export of terrestrial DOC. Under most riverine discharge conditions the estuary acts as a processer of terrestrial DOC. Immediately following extreme riverine discharge events, the estuary acts as a pipeline. In the last chapter we assessed the ability of estuarine phytoplankton and bacterial assemblages to use watershed dissolved organic nitrogen (DON) sources as a nutrient source. We used experimental DON addition bioassays to assess the impact of wastewater treatment facility effluent, chicken and turkey litter leachate, and river DON on phytoplankton growth. Chicken litter leachate was able to stimulate phytoplankton growth. Collectively, this research serves as a baseline for understanding the current FOM pool in the NRE and demonstrates how the OM pool may be changing in response to climatic and anthropogenic pressures.